Cocaine addiction (CA) is a common brain disorder, affecting ~ 1,000,000 adults in the US population, creating a significant public health problem because of its chronic nature and the associated morbidity and mortality. Although twin studies are consistent with an inherited component, it has been difficult to identify risk-increasing allels by studying DNA from blood or saliva. Retrotransposons (RTPs) are mobile DNA elements that are common in the human genome. In the past 5 years, data have accumulated to prove that neuronal embryogenesis is accompanied by activation of LINE1 (L1) RTPs to an unexpected degree, such that each developing neuron may accumulate multiple de novo L1 RTP genomic insertions, perhaps as many as ~80. This results in a substantial mosaicism within populations of CNS neurons. While most of these somatic de novo L1 RTP insertions will have little effect on neuronal function (perhaps because they occur in gene deserts or in large introns or in genes not required for that cell's function), some may interfere with normal neuronal activity because they have inserted into a gene needed by that particular neuron for normal function. If one or more functional L1 RTP insertion events occur early in CNS development, all the daughter neurons that derive from that neuronal precursor will also carry the L1 insertion, perhaps leading to a dysfunctional population of neurons destined to convey risk for CA. This application will leverage post-mortem brain tissue from CA patients, who have died from a cocaine overdose, and age/gender/ethnicity matched post-mortem brain tissue from controls. Laser capture microdissection will be used to harvest distinct populations of medial frontal cortex, amygdala and striatum (brain regions implicated in neuroadaptation in CA animal models). DNA from these neurons will be analyzed by high-throughput sequencing, to detect de novo L1 RTP insertions. Any de novo (not in the reference genome) L1 RTP insertion detected in a brain-expressed gene will be investigated for functional significance using standard assays of transcription. In this manner, it is expected that novel de novo somatic L1 RTP insertions, that increase risk for CA, will be discovered.